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  <title>Matrix multiplication — shape-safe</title>
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  <h1>Shape-safe matrix product</h1>
  <p>Edit <code>goodA</code>, <code>goodB</code>, <code>badA</code>, <code>badB</code>, then <strong>Run</strong>.</p>
  <pre id="out"></pre>
  <script>
    (function () {
      function canMultiply(a, b) {
        if (!a.length || !b.length || !a[0].length || !b[0].length) {
          return false;
        }
        var colsA = a[0].length;
        var rowsB = b.length;
        if (colsA !== rowsB) {
          return false;
        }
        var colsB = b[0].length;
        var i;
        for (i = 0; i < a.length; i++) {
          if (a[i].length !== colsA) {
            return false;
          }
        }
        for (i = 0; i < b.length; i++) {
          if (b[i].length !== colsB) {
            return false;
          }
        }
        return true;
      }
      function multiplySafe(a, b) {
        if (!canMultiply(a, b)) {
          return null;
        }
        var m = a.length;
        var n = a[0].length;
        var p = b[0].length;
        var result = [];
        var i, j, k;
        for (i = 0; i < m; i++) {
          result[i] = [];
          for (j = 0; j < p; j++) {
            result[i][j] = 0;
          }
        }
        for (i = 0; i < m; i++) {
          for (j = 0; j < p; j++) {
            for (k = 0; k < n; k++) {
              result[i][j] += a[i][k] * b[k][j];
            }
          }
        }
        return result;
      }
      var goodA = [[1, 2, 3], [4, 5, 6]];
      var goodB = [[7, 8], [9, 10], [11, 12]];
      var badA = [[1, 2], [3, 4]];
      var badB = [[5, 6, 7]];
      var lines = [];
      lines.push(JSON.stringify(multiplySafe(goodA, goodB)));
      lines.push(String(multiplySafe(badA, badB)));
      document.getElementById("out").textContent = lines.join("\n");
    })();
  </script>
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JavaScript Matrix Multiplication — Shape-safe (Try It)